1. Field of the Invention
This invention relates to coating and, more particularly, to direct application of electron irradiation and subsequent application of wave energy, preferably in the form of X-ray or short wavelength ultraviolet (deep UV) radiation.
2. Description of the Prior Art
Grebe, U.S. Pat. No. 3,849,136 entitled "Masking of Deposited Thin Films by Use of a Masking Layer Photoresist Composite," teaches provision of an aluminum mask for an actinic photoresist beneath it. A substrate is coated with layers of AZ-1350H photoresist (Shipley), separated by a layer of aluminum deposited by evaporation with the underlying resist maintained at room temperature. The upper layer also composed of actinic resist is exposed to a pattern by a conventional mask and is developed. Then openings in the aluminum are etched through the openings in the upper photoresist mask. Finally, all of the remaining upper layer of resist and the lower resist exposed by etching the openings in the aluminum mask are subjected to overexposure and then to development to provide undercut of the resist below the metal mask, providing a substantial overhang of the metal mask at all openings beyond the underlying photoresist of the lower layer of photoresist. Subsequently, the entire mask structure is removed by means of a lift-off process.
Franco et al., U.S. Pat. No. 3,873,361 entitled "Method of Depositing Thin Film Utilizing a Lift-Off Mask," shows depositing an organic polymer, actinic light sensitive photoresist layer which is baked to such an extent that it is no longer photosensitive (like a Bakelite layer). Then an inorganic, preferably metallic layer is deposited, composed preferably of copper, aluminum, chromium, glass, Si.sub.3 N.sub.4, or Al.sub.2 O.sub.3. Next, an upper layer of actinic photoresist is deposited, exposed through a mask and developed. The metal is etched chemically only through the new openings in the upper photoresist. Then, the newly formed integral metallic mask is used as a mask for the lower baked resist which is removed by sputter etching to produce a substantial overhang of the metallic layer through which a thin metallic film is evaporated. Then, the entire mask structure is removed by means of a lift-off process.
A similar lift-off process is shown in Bergrasse's publication entitled "Two-Resist Layers Lift-Off Process," IBM Technical Disclosure Bulletin, Vol. 16, No. 7, December 1973, pp. 2110-2111, which is intended to be used instead of an electron beam exposure system, using exposure of the two layers of actinic sensitive resist separately rather than electron sensitive resist, with an intermediate metallic (Cr) layer used as an integral mask prior to depositing through openings in the metal mask and a lift-off step. Note that the exposure through the metallic mask is achieved by using a variable angle of incidence, such as a rotating prism.
U.S. Pat. No. 3,447,924 of Trzyna et al. entitled "Aligning Method," employs a workpiece coated on both sides with visible light negative actinic photoresist, then on both sides with an X-ray sensitive emulsion. X-radiation is applied to one side passing through to the other side so that the image is developed on both sides, uncovering some parts of the visible-photoresist on both sides. The uncovered photoresist is then exposed on both sides. All X-ray resist is removed. The photoresist is developed and unexposed resist is removed. Then the workpiece is etched through the photoresist on both sides.
U.S. Pat. No. 3,874,916 of Livesay et al. entitled "Mask Alignment System for Electron Beam Pattern Generator," shows a substrate of glass coated with a thin film layer of chromium, which in turn is coated with an "electron resist." Then a thin film of a conductive metal such as Al is applied. Over the conductive metal is applied a thin film of a photosensitive actinic photoresist layer. The photoresist is exposed to a master mask (grid) and is developed. The conductive layer, which is then uncovered by developing photoresist, is etched through the openings in the resist. Subsequently, the electron sensitive resist can be exposed by scanning of the electron resist with an electron beam under computer control.
U.S. Pat. No. 3,679,497 of Handy et al. entitled "Electron Beam Fabrication System and Process for Use Thereof," describes exposure of an electron sensitive resist with an electron beam from a "photocathode" which is a body of material such as quartz substantially transparent to ultraviolet light, which has a pattern formed upon it of a metal which emits an electron beam in response to ultraviolet radiation. The exposed resist is then developed to open holes in the resist. Subsequently, other layers of electron sensitive resist can be added, and then a separate photocathode can be employed for each layer of electron sensitive resist. However, each layer of resist is exposed and developed before the next one is added.
None of the prior art offers a process which has the facility of using an electron beam exposure system which can be driven by a computer, plus the superior high aspect ratios which can be achieved by means of actinic exposure techniques and preferably X-ray or deep UV exposure techniques, which provide steep and straight side walls.
An object of this invention is to provide a method for producing masks for thin film fabrication processes employing resist materials, wherein the pattern can be generated effectively and the aspect ratio is high.
Another object of this invention is to provide a thin film resist with openings having straight side walls without undercut.
In accordance with this invention, a method is provided for forming a high aspect ratio pattern of openings in a resist structure. A first layer of a resist sensitive to actinic electromagnetic radiation in the visible light to X-ray range preferably from about 2 A to 5000 A in wavelength is applied to a substrate. Then an electron sensitive resist layer is applied over the actinic resist. The electron sensitive resist is then exposed to an electron beam operating to write a predetermined pattern in the resist, which pattern is subsequently developed. That pattern is then employed to form a corresponding pattern in a metallic thin film lying on the first layer. The metallic film is deposited on the first layer either before the electron resist layer or after the pattern has been formed in the electron sensitive resist. Next, the metallic thin film is employed as a mask for exposing the first layer of resist to radiation in the actinic range of frequencies. The first layer of resist is then developed.
In another aspect of this invention, the upper resist and lower resist layers can be composed of similar or identical resist materials and the exposure of the upper resist is performed with such an energy level that the lower layer of resist remains unexposed and the metallic layer, if in between resist layers, provides protection of the lower layer of resist. This is particularly appropriate for the case in which the metal film is formed by the subtractive process.
In an especially preferred embodiment, the lower resist is exposed by a source of X-ray or deep UV radiation.